This invention relates to the fabrication of microminiature devices and, more particularly, to mask structures for us in an X-ray lithographic system.
An advantageous mask structure for use in an X-ray lithographic system is disclosed in U.S. Pat. No. 3,892,973 issued to G. A. Coquin, J. R. Maldonado and D. Maydan. The structure disclosed there comprises a mask substrate made of a thin sheet of polyester film. The film is stretched over and bonded to a support member. The supported film constitutes a highly planar and durable substrate that is as dimensionally stable as the support member itself.
As disclosed in the aforecited patent, X-ray-absorptive elements definitive of a prescribed pattern are formed on the mask substrate. These elements may be made, for example, of a single layer of gold or platinum.
For some applications of practical interest, it has been demonstrated that it is advantageous to form the X-ray-absorptive elements of the mask from a trimetal system deposited on the substrate. One such system comprises a thin bottom layer of titanium utilized to enhance adhesion of a relatively thick layer of gold to the substrate. In addition, a layer of titanium on top of the gold serves as a masking layer during selective patterning of the gold.
Applicants have found that the top and bottom layers of a trimetal system of the type specified above are in practice typically deposited with stresses. In turn, these stresses have been observed to cause deleterious distortions of the substrate and of the high-resolution X-ray-absorptive pattern formed thereon. As a result of these distortions, X-ray mask structures of the type described were found to be unsatisfactory for some high-resolution applications, especially where each of a set of multiple masks had to be successively aligned with high precision with respect to a wafer.